Fluid, filter kit

Hot Tip:

Put a drop sheet under the work area

Performance Gain:

Proper shifting transmission

Complementary Modification:

Change transmission mount

One of the less common maintenance tasks performed on a car is changing the transmission fluid. Typically, you'll want to change the fluid every 50,000 miles or so along with the filter in the transmission. There are several versions of automatic transmissions used in the W123 but they are all 722 transmissions. Some transmissions are 722.3 and the new vehicles have 722.4 transmissions. The procedures for changing the fluid and filter are the same but some of the pictures may look slightly different from yours. What will be different are the parts you need, so please look at your owner's manual or run a VIN check to determine what transmission you have as this will determine the filter kit and fluid. We recommend that you make changing your oil and filter part of your regular maintenance.

Get the transmission fluid warm before draining it as this will help get all of the settlement and particulars in the fluid out, but use care that you do not burn yourself on the exhaust while working around it.

Begin by safely jacking up and supporting your vehicle. Please see our article on safely raising and supporting your W123. Place a large tarp or drip sheet under the car; have a large drain pan and lots of paper towels ready.

Figure 1

You will need to drain the old fluid from two locations on the transmission. In this view, you can see the main drain plug (yellow arrow) near the rear of the transmission pan on the left side and the drain plug located in the torque converter (red arrow, plug not visible). There has been a lot of discussion on whether or not to drain the torque convertor on a well working transmission; some people claim that this can upset sediment that can work its way into the valves. They therefore recommend only draining the pan fluid and after replacing the filter and fluid, run the transmission and then drain and refill the pan again a few times to get most of the old fluid out. It is up to you which method you use but we will show you both.

In order to access the drain plug in the torque converter, you'll need to rotate the engine until the plug is accessible through the window in bell housing. The easiest way is to place a 27mm socket on the front crankshaft pulley (red arrow) and turn the motor over until the plug is visible in the window.

Have a container ready to catch the fluid and so you can measure the amount that drains out. Also, always dispose of old fluids according to the regulations where you live. Never pour it in the street. Fluid will run and drip down the converter and bell housing and out the drain hole in the bottom of the bell housing, so be prepared for it (red arrow).

Remove the 5mm Allen plug on the pan (red arrow). Again; have a container ready to catch the fluid so you can measure the amount that drains. The transmission has a very narrow tolerance for the level of fluid in it and it is a good idea to add as much clean new fluid when you begin refilling as you drained old fluid out.

Use a 13mm socket and remove the six bolts holding the pan to the transmission. It is a good idea to leave two bolts (one in each corner) until you are ready to remove the pan. Even with draining there will be a surprising amount of fluid still in the pan. Place a hand on the pan and remove the final two bolts and then lower the pan carefully. Measure the amount of fluid in the pan. Depending on whether your transmission has been serviced before there may be zip ties attaching cables to the transmission; remove them if they are present.

Carefully pour the fluid into your catch container and inspect the pan for any metal shavings or pieces. A small amount of micro shavings can be normal; you should not find any pieces or metal build up in the pan (red arrow). Carefully clean the pan with a lint free cloth.

Now fill the transmission with Mercedes-Benz branded transmission fluid; it is critical that you use the original MBZ fluid in this application. The amount of fluid that you refill will be determined by whether you drained the torque convertor or not and you should be saving and measuring how much fluid you drained and begin by putting that much back into the transmission. Automatic transmissions a very sensitive to fluid levels. Safely run the car through all the gears to circulate the fluid through the transmission.

Check the dipstick for the transmission. You will want to measure the transmission level with the fluid hot and the vehicle running. Do not fill till the full marks (red arrow) with the transmission cold and the vehicle off.

You will need to monitor the temperature of the fluid as the car warms up. For this task, an infrared pyrometer is an invaluable tool. This tool allows you to point a laser beam at any given object and see the temperature. In this case, you would want to set the tool to Celsius and point it at the transmission pan until it reads roughly 80 degrees C (this will take a while). At this point, pull the dipstick out and read the level. If the level is correct, it will read between the lines for the 80 degree gradient on the dipstick. If it does not register, add more fluid until it does. You'll need to have the engine running and the transmission in park as you do this. Keep in mind that you will need to circulate the fluid between each time you fill by running the transmission through the gears. Once done, put the car back in park and re-check the level. It's a time consuming process, and you want to make sure that you get it just right. The fluid level MUST be correct or you could cause damage to the transmission.